1. Field of the Invention
This invention relates to a stencil mask used in semiconductor processes, and its manufacturing method.
2. Description of the Related Art
In semiconductor device manufacturing, when a plurality of MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors) differing in the conductivity type of channel are formed in a substrate, or when MOSFETs differing in threshold voltage are formed in a substrate, impurity ions are implanted into the wells, channels, or polysilicon layers. When ions are implanted, a stencil mask with openings is provided above a semiconductor substrate at a specific distance apart.
A stencil mask is used to project particles or electromagnetic waves onto a substrate to be processed. Particles include, for example, charged particles, such as electrons or ions, and neutral particles, such as atoms, molecules, or neutrons. Electromagnetic waves include light and X rays.
FIGS. 15A to 15D show conventional stencil mask manufacturing processes. A stencil mask is generally manufactured using an SOI (Silicon On Insulator) substrate 1000. Hereinafter, the processes of manufacturing a stencil mask will be explained.
FIG. 15A shows an ordinary SOI substrate 1000. The SOI substrate 1000 is formed by, for example, implanting oxygen ions into a silicon substrate and then annealing the resulting substrate at high temperature. A silicon oxide film 1002 is formed at a depth of several tens to several hundreds of nanometers from the top of the silicon substrate 1001. On the silicon oxide film 1002, a silicon thin film 1003 is formed.
Next, as shown in FIG. 15B, a resist (not shown) is applied to the surface of the silicon thin film 1003. The resist is processed by lithographic techniques, thereby forming a resist pattern. Thereafter, with the resist as a mask, the silicon thin film 1003 is etched anisotropically until the silicon oxide film 1002 is exposed. After openings 1004 are made in the silicon thin film 1003, the resist pattern is removed.
Next, as shown in FIG. 15C, a resist (not shown) is applied to the back of the silicon substrate 1001. The resist is processed by lithographic techniques, thereby forming a resist pattern. Thereafter, the silicon substrate 1001 is etched isotropically with chemical liquid, such as KOH. Specifically, the part of the silicon substrate 1001 where no resist pattern has been formed is etched isotropically until the silicon oxide film 1002 is exposed, thereby forming a support part 1001a. Thereafter, the resist pattern is removed.
Next, as shown in FIG. 15D, the silicon oxide film 1002 exposed in the process of FIG. 15C is processed from its back with chemical liquid, such as fluoric acid, thereby removing the silicon oxide film 1002 and exposing the silicon thin film 1003. In this way, a stencil mask 1005 with the openings 1004 is formed. A stencil mask with such a configuration has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-203806.
FIGS. 16A and 16B show the positional relationship between the stencil mask and a substrate to be processed in, for example, implanting ions.
As shown in FIG. 16A, the stencil mask 1005 is provided above a semiconductor substrate 1006. The openings 1004 in the stencil mask 1005 are caused to coincide with specific ion implantation regions 1007 of the semiconductor substrate 1006.
Next, as shown in FIG. 16B, impurity ions 1008 pass through the openings 1004 in the stencil mask 1005 and are implanted into the ion implantation regions 1007 of the semiconductor substrate 1006. Since there is no opening 1004 in the non-implantation region, impurity ions 1008 are cut off by the stencil mask 1005.
In this way, the stencil mask 1005 cuts off impurity ions 1008 repeatedly, allowing the cut-off impurity ions 1008 to be accumulated on the stencil mask 005. The repetitive collision of impurity ions 1008 damages the stencil mask 1005. In addition, since the silicon thin film 1003 in which the openings 1004 are made is thin, it is expected that the film will be deformed due to a load caused by gravity or inertial force during transportation or movement. The deflection strength of the silicon thin film depends on physical properties, including Young's modulus, and the area of the thin film region. Generally, the strength of the film is proportional to the cube of the film thickness. Therefore, making the film thickness of the stencil mask 1005 thicker enables the strength to increase, which prevents the stencil mask 1005 being deformed as a result of the ion implantation described above.
On the other hand, as described above, the openings 1004 in the stencil mask 1005 are made by processing the SOI substrate 1000 by anisotropic etching. Therefore, the accuracy of the openings 1004 in the stencil mask 1005 depends on the material and thickness of the film to be processed. Generally, the processing accuracy depends on the ratio of the opening dominions to the depth direction (film thickness). This is called the aspect ratio. Accordingly, when a film is thick, the opening dimensions of an opening made in the film become large, making microfabrication difficult. Conversely, when a film is thin, an opening with small opening dimensions can be made. Therefore, making the film thickness of the stencil mask 1005 thicker to increase the strength makes it difficult to make microscopic openings 1004.
Furthermore, in the process of manufacturing semiconductor devices, there may be a case where ions are implanted diagonally or a stencil mask is used in a reduction exposure process. In this case, particles are directed diagonally at the stencil mask provided in parallel with the semiconductor substrate.
FIG. 17 shows a case where particles are directed diagonally by use of the stencil mask 1005 with openings 1004 processed perpendicularly to the surface by anisotropic etching. In this case, there arises a problem: part or all of the particles passing through the openings 1004 are obstructed by the sidewalls of the openings 1004 in the stencil mask 1005, and therefore particles do not reach the semiconductor substrate. Such a phenomenon is called shadowing.
The influence of shadowing will be explained by reference to FIG. 17. Suppose charges particles are directed at an angle of θ at a stencil mask for forming an implantation region of a substrate to be processed. If the film thickness of the stencil mask is T, each of the opening dimension of the stencil mask and the dimension of the implantation region of the substrate to be processed is S1, and the dimension of the region of the substrate into which charged particles are implanted is S2, S2 is expressed by the following equation:S2=S1−T·tan θ
As seen from the above equation, of the region S1 into which ions are supposed to be implanted, only the region expressed by T·tan θ is cut off by the stencil mask. The larger the incident angle becomes, the larger the cut-off region becomes. In addition, the thicker the film thickness T of the stencil mask, the larger the cut-off region becomes. For this reason, it is impossible to secure the desired implantation region S1. Consequently, when the silicon thin film is made thicker, to increase the strength of the stencil mask, the percentage of cut-off particles, caused by shadowing, becomes larger. To overcome this problem, a stencil mask which enables microscopic openings to be made, while maintaining adequate strength, and a method of manufacturing the stencil mask, are required.